1. Field of the Invention
The present invention relates to an ink jet printing apparatus that carries out color printing using a plurality of inks, and an ink jet print head used in this ink jet printing apparatus. Specifically, the present invention relates to an ink jet printing apparatus that carries out printing by causing a print head to execute a main scan, the print head comprising a plurality of nozzle rows each of which ejecting different colors, extending in a predetermined direction and arranged along a main scanning direction orthogonal to the predetermined direction, as well as an ink jet print head used in this ink jet printing apparatus.
2. Description of the Related Art
Ink jet printing apparatuses have been spreading rapidly because of their relatively small sizes, low noise, low costs required for color printing, and the like. Furthermore, recent ink jet printing apparatuses can achieve high image quality and operate at high speeds: they can output an A4-sized image of a photographic quality within about one minute. This is mainly due to technical improvements in print heads, ink, and print media, but another likely reason for the improved performance of these apparatuses is the mechanical control of the main body of the apparatus and the like.
The recent ink jet printing apparatuses are commonly of a general type that prints a print medium by alternately repeating a printing operation of moving a print head on the print medium in a main scanning direction (this operation will herein after be referred to as a “main scan”) and ejecting ink during this scan and a sheet feeding operation of conveying the print medium by a specified amount in a direction perpendicular to the main scanning direction (this operation will hereinafter be referred to as a “sub-scan”). The time required by these serial type ink jet printing apparatuses for printing varies significantly depending on, for example, the scan control of the print head.
The configuration of the print head in the serial type color printing apparatus is roughly classified into two types.
In a first type of print head (vertically arranged head), a large number of nozzles are linearly arranged in a sub-scanning direction as shown in FIGS. 11A and 11B. In FIG. 11A, rows of color nozzles through which yellow, magenta, cyan, and black inks, respectively, are ejected are arranged in a line in a sheet feeding direction so that the rows do not overlap one another. Further in FIG. 11B, a row of black nozzles through which black ink is ejected is constructed separately from color nozzle rows through which color inks are ejected.
For example, if the arrangement shown in FIG. 11B is used to execute color printing, the color inks are ejected in accordance with print data. However, a single main scan only forms ink dots of each color on a print medium at different positions. Then, to form secondary colors, dots of secondary colors must impact the same area on the print media. Accordingly, the sheet is fed by an amount corresponding to a row of nozzles, i.e. the length shown at h in the figure. Therefore, the print head scans the same area on the print medium about three times. Further, the nozzle rows always scan the same area on the print medium in the order of cyan, magenta, and yellow regardless of the scanning direction of the print head. Accordingly, in forming blue, red, and green, what is called “secondary colors”, the colors are allowed to overlap one another in a fixed order regardless of the scanning direction of the print head (e.g. forward direction or back direction). For example, to form a blue image, cyan is printed first and magenta is then printed on it. Consequently, the use of the illustrated print head prevents the colors from being non-uniformly printed colors even if an image is printed by alternately scanning the print head forward and backward, i.e. even if what is called “bidirectional printing” is carried out.
However, with the illustrated head arrangement, if the number of nozzles is increased to increase printing speed, then the print head becomes longer to increase the size of the head or the whole apparatus. In another case, a method of holding a print medium in a printing section tends to be complicated. This disadvantageously increases the costs of the print head or the whole apparatus.
In a second type of a print head (horizontally arranged head), ink ejecting sections, from which black ink, cyan ink, magenta ink, and yellow ink, respectively, are ejected, are arranged in parallel with the main scanning direction, for example, as shown in FIG. 12. With a print head in this form, a single scan allows all the color inks to be ejected to the same area on a print medium in accordance with image data.
It is assumed that ink is ejected from the print head not only during a forward scan (the direction of arrow A in the figure) but also during a backward scan (the direction of arrow B in the figure) in order to increase the printing speed. Then, in forming blue, red, and green, what is called “secondary colors”, the order in which the colors are allowed to overlap one another differs between the forward scan (the direction of arrow A in the figure) and backward scan (the direction of arrow B in the figure) of the print head. As a result, tints vary among the different scan operations to make the colors non-uniform, thus significantly degrading the image. The non-uniformity is particularly significant in a high-gradation image such as solid printing.
On the other hand, a multipass printing method is available for improving the quality of the image. With this method, differences among the nozzles characteristic of the head are reduced by printing the same area using two or more scan operations. In this case, the distance a print medium is moved during a single operation is equal to or smaller than the length of the head. Accordingly, the multipass printing method requires a larger number of scan operations than the other methods and thus generally requires more time for printing. Thus, in order to reduce the printing time, reciprocatory printing can be effectively used even if an image is formed using the multipass printing method. On the other hand, the color non-uniformity(the color variations) resulting from a difference in color overlapping order between forward printing and backward printing is not completely avoided even when the multipass printing method is used. It is possible to increase the number of passes to make the color non-uniformity almost unnoticeable. However, more time is required for printing. Consequently, the reciprocatory printing, which is used to reduce the time required for printing, becomes meaningless.
Further, as shown in FIG. 13, to form a higher-quality image, a 6-color print head may be used in which ink ejecting sections for black ink, cyan ink, magenta ink, and yellow ink, as well as light cyan and light magenta ink are arranged parallel with one another. Even with this 6-color print head, color non-uniformity may be caused by a difference in color overlapping order between forward printing and backward printing, as with the 4-color print head in FIG. 12.
As described above, there are two types of print head arrangements suitable for an ink jet printing apparatus that carries out color printing: an arrangement (vertically arranged head) in which the nozzle rows for the respective ink colors are arranged in a line and an arrangement (horizontally arranged head) in which the nozzle rows are arranged parallel with one another. The parallel arrangement is suitable for high-speed printing. However, with this arrangement, color non-uniformity may be caused by a difference in color overlapping order between forward printing and backward printing.
To solve this problem, Japanese Patent Application Laid-open No. 2001-171119 describes a print head in which nozzle rows for color inks are arranged symmetrically in the main scanning direction. According to this invention, ink is allowed to impact print media in the same order at all times by changing ejecting nozzle rows between a forward scan and a backward scan. This arrangement enables to resolve color non-uniformity resulting from a difference in ink impacting order.
However, even an ink jet printing apparatus using a print head in which nozzle rows are symmetrically arranged has the following problem.
With this arrangement, a plurality of nozzle rows are required for the same color, thus increasing the size of the head. Furthermore, an electric or mechanical system in this arrangement requires more wires than the other conventional electric or mechanical systems. This increases the costs of the apparatus. Furthermore, if the above described arrangement is provided using six color inks including light cyan and light magenta, 12 (6 colors×2) nozzles are required. As a result, the sizes of the head and the whole apparatus increase sharply, and an ink channel arrangement becomes very complicated. It is also likely that sufficient suction is impossible during a suction and recovery process. This also degrades the reliability of the apparatus.